This application relates generally to fiber optics and more specifically to fiber optic switching devices.
For high bandwidth fiber optics communication systems, an important functional need is to add or drop particular channels at a particular node. For example, there is a need to add optical signals from various optical channels/nodes onto a trunk line and drop optical signals from a trunk line onto various channels. Adding and dropping should occur with low insertion loss and low crosstalk. That is, an effective optical add drop multiplexer should add a significant fraction of the light from an optical channel to an intended trunk line and add substantially none of the light to unintended channels. Light coupled to unintended channels is referred to as crosstalk and is typically expressed in terms of attenuation. Attenuation if expressed in decibels or dB, and xe2x88x9250 dB is generally considered a target performance level. Moreover, an effective optical add drop multiplexer should drop optical signals, such that a significant fraction of the light is dropped from a trunk line into a designated optical channel and substantially none of the light from the trunk line should be coupled to unintended channels. A trunk line can be any optical transmission line that carries multiple optical signals such as the optical rings of a SONET network, or long or short haul transmission lines. Optical channels to which optical signals are dropped or from which optical signals are added to a trunk line, include for example end user channels or other networks. Such optical channels may carry single optical signals or multiple optical signals to be further distributed.
A beneficial function for add drop multiplexing is configurability. Configurability provides for optical signals to be either dropped from a trunk line at a node, or to be kept on the trunk line and bypass the node. Such features provide numerous benefits to optical networking. For example, trunk lines can have nodes that can be bypassed until a node is ready for use. This is particularly valuable when new optical networks are installed but individual end users are not prepared to hook up to the optical network. Such end users can be bypassed and later switched into the optical network quickly and inexpensively without the necessity of additional hardware installation.
A further beneficial function for add drop multiplexing is the preservation of the intensity of the various polarizations of an optical signal as it traverses the various components of an add drop multiplexer. In other words, an incoming beam should not be split according to its variously polarized component signals. Preserving intensity of the various polarizations of the incoming beam serves to effectively lower insertion loss and crosstalk. In addition to the functional performance characteristics mentioned above, it is desirable that the add drop be reliable, compact, and inexpensive.
Prior art optical add drop multiplexers include such devices as, non-configurable add drop multiplexers, star coupler devices, as well as other devices. While all of these technologies have been demonstrated for optical add drop multiplexing, their cost to manufacture and use can be relatively high. For example, in optical networks using non-configurable add drops, the bypassing of a node usually requires the installation of costly optical regeneration hardware. Regeneration hardware needs to be added at non-used nodes to add optical signals back onto a trunk line, because the optical signals were necessarily dropped due to a lack of configurability. In a further example, optical networks having star coupler devices, often require the use of optical regeneration after multiplexing or demultiplexing because of relatively high signal attenuation associated with such devices. Thus, considerable efforts are still ongoing to develop an all optical add drop multiplexer characterized by configurability, low loss and crosstalk, high speed and reliability, small overall size, and low cost.
As is well known, typical single-mode fiber optics communications are at wavelengths in the 1300-nm and 1550-nm ranges. The International Telecommunications Union (ITU) has defined a standard wavelength grid having a frequency band centered at 193,100 GHz, and other bands spaced at 100 GHz intervals around 193,100 GHz. This corresponds to a wavelength spacing of approximately 0.8 nm around a center wavelength of approximately 1550 nm, it being understood that the grid is uniform in frequency and only approximately uniform in wavelength. Implementation at other grid spacings (e.g. 25 GHZ, 50 GHz, 200 GHz, etc.) are also permitted. This frequency range and frequency spacings provides an enormous bandwidth for use in audio, video, audio-video as well as other communications needs such as the Internet and provides an impetus to develop optical technologies to exploit such bandwidth, such as all optical add drop elements for configurable add drop multiplexers with the previously listed characteristics.
The present invention provides a wavelength add drop element (WADE) for use in configurable wavelength add drop multiplexing.
The WADE includes first, second, and third reflectors. The first reflector, referred to as the configurable reflector, is disposed to intercept the incoming light (having at least first and second wavelengths) traveling along an input path. The first reflector has a first state in which it transmits the incoming light along a first transmitted path and a second state in which it reflects the incoming light along a first reflected path. The second reflector is a wavelength-selective reflector that reflects light of at least the first wavelength and transmits light of the second wavelength. The third reflector reflects at least light of the first wavelength. The first, second, and third reflectors are oriented so that when the incoming light is transmitted along the first transmitted path, the light of the first wavelength is reflected by the second and third reflectors to travel along second and third reflected paths. The third reflected path intersects the first reflector at an angle such that the light of the first wavelength is transmitted by the first reflector and continues on the first reflected path. Light of the second wavelength is transmitted by the second reflector along a second transmission path that is distinct from the first transmission path. The first, second, and third reflectors are further oriented so that the first transmitted and reflected paths are more than 45 degrees from the normal to the first reflector, and the second transmitted path is less than 22.5 degrees from the normal to the second reflector.
In some embodiments the first reflector is located in a routing region bounded on first and second sides by a first transparent material having a refractive index greater than 1. Further, the first reflector includes a body of a second transparent material having a refractive index greater than 1 disposed in the routing region. The first reflector body has a contracted state at a first temperature and an expanded state at a second temperature. The contracted state defines an air gap disposed in the path of the incoming light traveling along the input path so as to cause the incoming light to be deflected onto the first reflected path through total internal reflection. The expanded state removes the air gap disposed in the path of the incoming light traveling along the input path so as to allow the incoming light to pass through the body of transparent material and travel along the first transmitted path.
In some embodiments the first transparent material is silica and the second transparent material is an elastomeric material.
In some embodiment the incoming path, the first and second transmitted paths, and the first, second, and third reflected paths are defined by waveguide segments in a planar waveguide structure. WADEs according to different embodiments of the invention are readily incorporated in a variety of configurable wavelength add drop multiplexers.